A Raman scattering study of structural changes in LaMn_1-xCo_xO_3+δ system

We present results of Raman scattering studies on LaMn_1-xCo_xO_3+δ over a wide range of doping content (x = 0.1-0.75) and temperature range of 20-300 K. Powder X-ray diffraction patterns show that there is a structural change from orthorhombic to rhombohedral at x = 0.5 as x increases. Raman spectra of all LaMn_1-xCo_xO_3+δ samples show peaks near 260, 500, and 650 cm^-1. However, the Raman spectra are not drastically different from each other across the structural phase transition at x = 0.5. On the other hand, the peak frequencies of the modes near 260 and 500 cm^-1 as functions of Co content (x) show slope changes at x = 0.5. The full-width at the half-maximum (FWHM) of the mode near 650 cm^-1 as a function of Co content (x) shows minimum at x = 0.5. Normally, larger values of FWHM are expected at near x = 0.5, if the mode were affected by the structural disorder at the phase boundary. Therefore, it is likely due to lowest charge concentration at x = 0.5, which results in lowest screening effect. This is consistent with the fact that the intensity of the phonons is strongest at x = 0.5. As the temperature decreases, the two peaks near 500 and 650 cm^-1 of different Co contents, related with octahedral distortions, are found to shift to lower frequencies unlike the usual temperature behavior. However, no abrupt change in the peak frequencies and the FWHM is observed across measured temperature range, regardless of the Co content.